Liquid crystal display elements are being used in not only watches and electronic calculators, but also various measuring devices, panels for automobiles, word processors, electronic notebooks, printers, computers, televisions, clocks, advertisement display boards, etc. Typical examples of liquid crystal display mode include a TN (twisted nematic) mode, an STN (super twisted nematic) mode, a VA (vertical alignment) mode characterized by vertical alignment, and an IPS (in-plane-switching) mode/FFS (fringe-field-switching) mode characterized by horizontal alignment, the VA mode and the IPS mode/FFS mode using thin-film transistors (TFTs). It is desirable that liquid crystal compositions used in these liquid crystal display elements be stable against external factors such as moisture, air, heat, and light, be in a liquid crystal phase over a temperature range as wide as possible around room temperature, have a low viscosity, and operate at a low driving voltage. Furthermore, liquid crystal compositions are constituted by several to several tens of compounds so as to optimize dielectric anisotropy (Δ∈) and/or birefringence (i.e., refractive index anisotropy) (Δn), etc. for respective display elements.
In vertical alignment displays, a liquid crystal composition having a negative Δ∈ is used. In horizontal alignment displays such as a TN mode, an STN mode, an IPS mode, or the like, a liquid crystal composition having a positive Δ∈ is used. Recently, a driving mode has been reported with which a liquid crystal composition having a positive Δ∈ is aligned vertically in the absence of applied voltage, and an IPS/FFS mode electric field is applied to perform display. Thus, the need for liquid crystal compositions having a positive Δ∈ has been further increasing. Furthermore, low-voltage driving, high-speed response, and a wide operation temperature range are desirable in all driving modes. Specifically, a positive Δ∈ having a large absolute value, a low viscosity (η), and a high nematic phase-isotropic liquid phase transition temperature (Tni) are desirable. From the viewpoint of setting of Δn×d, which is the product of Δn and a cell gap (d), it is also necessary to adjust the Δn of a liquid crystal composition to be in an appropriate range in accordance with the cell gap. In addition, high-speed response is important in the case where a liquid crystal display element is applied to a television or the like, and thus a liquid crystal composition having a low rotational viscosity γ1 is desirable.
Liquid crystal compositions have been disclosed in which compounds represented by formulae (A-1) and (A-2), which are liquid crystal compounds each having a positive Δ∈, are used as components thereof (PTL 1 to PTL 4). However, a sufficiently low viscosity has not been realized by any of these liquid crystal compositions.
